How to safely synching my iPhone 3GS into multiple computer

I got iphone 3GS that i'd like to synch into 3 computers (1 work lappy 1 my home pc and the other one my home PC) all just for transferring the music INTO my iphone not the other way around, can I do this without losing my iPhone music or data ?

Sure. You just have to be careful. Presumably, you want it to sync data with only ONE of these machines. If so, set that one up to sync data, and, in iTunes on each of the other two machines:

Connect the iPhone
Go to "Info" in iTunes
Uncheck everything on the "info" page, so that no information is sync'd.

On ALL THREE MACHINES, each in turn:
Connect the iPhone
Go to "Summary" in iTunes
Check the box that says "manually manage music and videos."
Click Apply

Now, NONE of the machines will "automatically" copy music to the iPhone, or sync the iPhone to its own library. Instead, you now move music to the iPhone by going to "Music" (under Library) in iTunes, and selecting and dragging songs from the library to the iPhone (which is listed under "Devices.") You can select multiple songs to copy using the usual Control-key/arrow and shift-key arrow combinations that work in most Windows applications.

You would delete songs from the iPhone by opening up the iPhone's contents (in iTunes) by clicking the little triangle next to the iPhone (under Devices), and choosing music from the menu that expands under it, and then deleting the songs you no longer want on the phone. This won't delete the songs from the PC's -- just from the phone.

But as soon as you connect the iPhone to a PC where the "manually manage music and videos" box is NOT checked, you will wipe out all the music on the iPhone, and replace it with the music in that machine's iTunes library...so be careful!

What @Akahan above describes will work but its not a true SYNC.. in that it doesnt actually sync. What you want cant be done. The itunes <-> Iphone relationship is 1:1 in that any music in your itunes library is copied to a device, anything which isnt in the library is removed from the phone. Based on this you cant do a 2:1 or 3:1 sync with your phone.

Right, "Sync" generally means, "Make the iPhone contain exactly what the library on this computer contains." By definition, sync'ing will not accomplish what the original poster wants, because it will always remove from the iPhone everything that is not in the library on the machine the iPhone is not currently connected to.

Indeed, but 3 way syncing is possible in theory, just 'combine' the contents of any syncing pair... would keep 3 machines and 1 phone in sync. Unfortunately apple dont allow it, which is a shame as i would love to be able to keep MacPro - Laptop and Phone in sync...

What do you mean "cannot delete some of the content of my music"? Cannot delete it from what, the phone, the PC? How are you trying to delete it? You should be able to delete whatever you want.

Also, as mentioned above, you will not be able to "sync" music without deleting the music you previously synced from another machine on the iphone. You need to change all the machines to manually manage music, and then COPY the music (not "sync") to the iphone.

You should read OS supplied guidelines before developing. I can't stress that enough. The guidelines will help you understand the reasons mobile app developers do what they do. Apple is very particular when they review appstore submissions.

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